In general, radial tires are provided with tread reinforcing belts. For such belts, a compact cord having a 1X3 structure has been widely used.
Such a 1X3 compact cord is composed of three steel monofilaments which are twisted together compactly. That is, no gap is formed between the adjacent filaments, as shown in FIG. 6. Therefore, a closed space (H) is formed at the center of the cord (A), and topping rubber (B) can not penetrate into this central space (H). As a result, the central part of the surface of the monofilament (F) is not covered by the topping rubber (B). The uncovered part is easily corroded by water which penetrates the tire tread portion from its damaged part. And the corrosion is liable to spread quickly through such a space (H), which greatly reduces the tire durability.
In order to avoid exposure of the steel monofilament surface, a loose cord has been proposed, wherein, as shown in FIG. 7, straight steel monofilaments (F) are loosely twisted together. Therefore, penetration of topping rubber can be improved and the surfaces of the filaments can be covered easier by the topping rubber because, when the cord tension is small, the gaps (G) between the filaments are sufficiently large and the topping rubber can easily penetrate the cord.
However, because of a very large elongation of such a loose cord under a small load, when loose cords are assembled in a raw tire as a belt and the tire inside is pressurized in the vulcanizing process, the belt tends to be elongated or deformed unevenly, which deteriorates tire uniformity and also decreases belt rigidity. As a results, steering stability, rolling resistance, tire life and the like are greatly decreased.
Further, if a tension is applied to the loose cord during rubberizing of the cord, the gaps between the filaments becomes narrow or closed, which results in the same problem of less rubber penetration.